# [GAP Forum] Debugging advice needed

Mathieu Dutour Mathieu.Dutour at ens.fr
Fri Jul 25 12:13:28 BST 2008

Dear all,

I experienced a bug in a computation:
----------------------------------------------
Error, <elm> lies not in group defined by <pcgs> called from
ExponentsOfPcElement( pcgs, pcgs[n] ^ p ) called from
PermpcgsPcGroupPcgs( pcgs, IndicesEANormalSteps( pcgs ), false ) called from
IsomorphismPcGroup( G ) called from
MinimalGeneratingSet( G ) called from
SmallGeneratingSet( GrpStab ) called from
...
you can 'quit;' to quit to outer loop, or
you can 'return;' to continue
brk> GRP:=Group(GeneratorsOfGroup(GrpStab));
Syntax error: warning: unbound global variable in *errin* line 1
GRP:=Group(GeneratorsOfGroup(GrpStab));
^
<permutation group with 16 generators>
brk> SmallGeneratingSet(GRP);
[
(1,2)(3,7)(4,8)(5,9)(6,10)(11,13)(12,15)(14,16)(19,20)(21,22)(23,25)(24,26),
(1,2)(3,7)(4,8)(5,9)(6,10)(11,13)(12,15)(14,16)(17,18)(19,20)(21,22)(23,
25)(24,26)(27,28)(29,30), (1,2)(3,7)(4,8)(5,9)(6,10)(11,13)(12,15)(14,
16)(17,31)(18,32)(19,25)(20,23)(21,26)(22,24)(27,30)(28,29),
(1,10)(2,6)(3,8)(4,7)(17,18)(19,26)(20,24)(21,23)(22,25)(27,28)(29,30)(31,
32), (1,11)(2,13)(4,8)(5,10)(6,9)(14,16)(17,24,18,26)(19,27)(20,28)(21,
31)(22,32)(23,29,25,30), (1,16)(2,14)(3,10)(4,13)(5,15)(6,7)(8,11)(9,
12)(17,32,18,31)(19,29,20,30)(21,24)(22,26)(23,27)(25,28) ]
brk> GRP=GrpStab;
true
brk>
-------------------------------------------------------
So, "GRP" and "GrpStab" have different behavior but are tested to
be equal. What could be the cause of the problem?
I never encountered this since so far in my years of use of GAP, when
a program stops, the reason is clear.

The problem is reproducible but with many subprograms to put before
getting to the problem and so I can't put it here. If that is relevant
the problem show up on gap4r4p10 with gcc compilers on x86 32 bits
machine.